ELI5: Where do smells go when they ?go away??

Especially if it?s an enclosed space with no ventilation, and the smell eventually goes away. Where does it ?go??
Edit: more specifically, how does it dissipate?
A crude example: My husband destroys our bathroom. We unfortunately don?t have a built in fan in the bathroom. We just close the door so the smell doesn?t get into the rest of the house. In an hour, we open the door, the smell is gone. Where did the smell go?


Source.